connect string trouble

I am going to learn a little about Oracle, so that I can finish my school in spring'98
Therefore I have purchased the book : Oracle Unleashed, accompanied by a

cd-rom with Personal Oracle 7 for windows 95 and oracle developer 2000 trial versions.
Personal Oracle and the PO7 Navigator works fine, as well does everything in that package.
I am not on a network, Yet I cant seem to be able to connect from sql*plus 3.1, installed alongside developer 2000, to the same database that I have no trouble accessing with sql*plus 3.2 which has been installed alongside PO7.

Has this somthing to do with the connect string ?

If I try to run for example the SQL-script "Common Build" from "Developer 2000 Administration " I get an error ORA-03121 with a danish text, saying that no error text is found.

Oracle.Ini is in place as well as the entry in win.ini.

Please Help me : I would very much like to be able to build my first data entry form in Forms 4.5.

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Joergen Gaardsted Joergensen